Mesoporous Carbon Nanofiber Using

Article pubs.acs.org/JPCC

Fabrication of Macroporous/Mesoporous Carbon Nanofiber Using CaCO3 Nanoparticles as Dual Purpose Template and Its Application as Catalyst Support Hua Liu,†,‡ Chang-Yan Cao,† Fang-Fang Wei,†,‡ Yan Jiang,†,‡ Yong-Bin Sun,†,‡ Pei-Pei Huang,†,‡ and Wei-Guo Song*,† †

Beijing National Laboratory for Molecular Sciences (BNLMS) & Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: The 1D hierarchical macroporous/mesoporous carbon nanofibers were prepared via electrospinning using PAN as carbon precursor and commercially available nanoCaCO3 as dual purpose template. During the carbonization process, nano-CaCO3 template decomposed and released CO2 to develop mesopores, and macropores were generated by subsequent acid removal of the as-formed CaO nanoparticles. This method is facile and low cost, allowing high-yield production of 1D hierarchical porous carbon nanofibers. The unique macro-/mesoporous structure of the nanofibers makes them a good support for anchoring palladium nanoparticles; the as-prepared catalyst shows high activity in various Suzuki cross-coupling reactions.

1. INTRODUCTION Catalyst support is a key component in an effective heterogeneous catalyst. Pore structures of catalyst supports play a vital role in the diffusion of reaction species.1−5 While micropores are widely utilized in gas phase reactions, mesopores are perhaps more effective in liquid phase reactions. Mesopores provide high surface areas and even shape selectivity for guest molecular in liquid phase catalysis.6 However, conventional mesoporous silica materials have considerable diffusion barrier for reaction species with long diffusion length as well as the lack of hierarchical pore structures.7 We propose that carbon nanofibers with hierarchical macroporous/mesoporous pores with shortened diffusion length will overcome above difficulty. Electrospinning is a low cost yet an effective method to produce continuous carbon nanofibers. The diffusion length along the cross section is usually less than 100 nm on a carbon nanofiber.8−12 However, the conventional electrospinning technique can only produce solid nanofibers; thus, post-treatment methods have been proposed to generate desirable pores inside the carbon nanofibers using either hard or soft template.11,13−19 Macropores, mesopores, or micropores were generated by using these methods. However, these methods generated only one type of pore structure, i.e., macropores, micropores, or mesopores only. For catalyst support, it is particularly appealing to synthesize macro- and mesoporous carbon nanofibers because the hierarchical porous structures have the advantages of each © 2013 American Chemical Society

class of the pores for optimized transport and diffusion pathways.20−24 So far, a method to prepare carbon nanofibers with well-controlled hierarchical pore structures is still lacking. In this study, we found out that CaCO3 nanoparticles could serve as a dual purpose template to generate hierarchical macro/mesopores inside carbon nanofibers. 1D carbon nanofibers composed of hierarchical macro- and mesoporous structures were produced by coelectrospinning polyacrylonitrile (PAN) and commercially available CaCO3 nanoparticles followed by thermal treatment and acid leaching. CaCO3 nanoparticles acted as a dual purpose template. During the carbonization, decomposition of CaCO3 resulted in CO2 gas, which generated mesopores when being discharged out of the nanofibers, and then the acid washing removed the as-formed CaO nanoparticles to generate macropores. The sample combines the advantages of interconnected macro- and mesopores and has shown excellent performance as catalyst support for Pd nanoparticles in liquid phase catalysis.

2. EXPERIMENTAL SECTION 2.1. Fabircation of Nano-CaCO3−PAN Composite Nanofibers by Electrospinning. The composite precursor nanofibers were prepared by a common electrospinning Received: August 7, 2013 Revised: September 17, 2013 Published: September 19, 2013 21426

dx.doi.org/10.1021/jp4078807 | J. Phys. Chem. C 2013, 117, 21426−21432

The Journal of Physical Chemistry C

Article

Scheme 1. Schematic Illustration of the Fabrication Process of the Hierarchical Macro/Mesoporous Carbon Nanofibers

solution. 1.0 M NaOH was used to adjust the pH value to 8. Then 400 mg of NaCOOH·2H2O was added to the suspension and maintained at 80 °C for 2 h under magnetic stirring. The catalyst was collected by filtration and washed with distilled water followed by drying in the vacuum oven at 60 °C overnight. The as-prepared catalyst was denoted as Pd/GCNFs-900. The Pd loading of the Pd/G-CNFs-900 was 7.6 wt % determined by ICP analysis. 2.4. Catalytic Test. Suzuki cross-coupling reactions were performed to test the catalyst. In a typical experiment, 5 mg of Pd/G-CNFs-900 catalyst, aryl halide (0.5 mmol), phenylboronic acid or 4-vinylphenboronic acid (1.0 mmol), K2CO3 (1.5 mmol), and pentamethylbenzene (0.5 mmol, as internal standard for high-performance liquid chromatography (HPLC) analysis) were added to the solvent (10 mL). The reactions were carried out at reflux condition for a definite time. Then the mixture was separated quickly by centrifugation, and the liquid was analyzed by HPLC. 2.5. Characterization. The structures and morphologies of the products were characterized by TEM (JEOL-1011) and SEM (JEOL 6701F). The nitrogen adsorption and desorption isotherms were measured on Quantachrome Autosorb AS-1 Instrument. The pore size distributions were derived from the desorption branches of the isotherm with the Barrett−Joyner− Halenda (BJH) model. The macroporosity was recorded by mercury intrusion porosimetry using a Micrometrics Autopore IV 9500 porosimeter. Thermal decomposition and the oxidative stabilization process were investigated by thermogravimetric (TG)/differential thermal analysis (DTA) (TG/DTA6300, America), which were performed between ambient and 1000 °C with heating rate of 10 °C min−1 under the flowing Ar. Pd content was characterized by ICP-AES (Shimadzu ICPE-9000). The conversions of reagents were measured using a HPLC (Shimadzu LC-10 AVP Plus).

method. In a typical procedure, 1.5 g of nano-CaCO 3 (purchased from Xintai Nano Co., China) was dispersed in 10 g of dimethylformamide (DMF) by sonication for 30 min, then 1.5 g of PAN was added, and the mixture was heated at 80 °C for 2 h under stirring to form a homogeneous suspension. The milky suspension was loaded into a plastic syringe equipped with a 20 gauge needle made of stainless steel. The needle was connected to a high-voltage supply. A piece of grounded aluminum foil was placed 15 cm below the tip of the needle to collect the nanofibers. The voltage was set at 25 kV, and the solution feeding rate was 1.5 mL h−1. The white nonwoven mat composed of composite nanofibers was collected on the aluminum foil collector. 2.2. Preparation of Hierarchical Porous Carbon Nanofiber. The as-collected electrospun composite nanofibers were peeled off from the aluminum foil and placed into a horizontal tube furnace for heat treatment. The nonwoven mat was heated to 280 °C in air at a rate of 2 °C/min and maintained for 2 h for stabilization; then the sample was heated up to 900 °C at a rate of 4 °C/min and kept for 2 h under argon gas flow (60 mL/min) for carbonization of PAN and pyrolysis of CaCO3 to CaO. The product was cooled to room temperature under an Ar atmosphere, and carbon nanofibers embedded with nano-CaO were obtained. In order to remove the CaO nanoparticles, the obtained CaO-carbon nanofibers were immersed in 2 M HCl acid for 0.5 h and then rinsed with deionized water thoroughly. The obtained porous carbon nanofibers were denoted as G-CNFs-900. For comparison, the porous nanofibers G-CNFs-650 were prepared by a similar method except that the carbonization temperature was 650 °C, which was lower than the pyrolysis temperature of CaCO3. The nano-CaCO3 was also removed by 2 M HCl for 2 h after carbonization of CaCO3−PAN nanofibers, and the G-CNFs650 were then obtained. 2.3. Synthesis of Pd/G-CNFs-900. Typically, 100 mg of G-CNFs-900 was dispersed in 20 mL of deionized water under ultrasonication for half of an hour, and then 10 mL of Na2PdCl4 solution containing 30.7 mg of Na2PdCl4 was added into the

3. RESULTS AND DISCUSSION The overall preparation procedure for hierarchical porous carbon nanofibers is illustrated in Scheme 1, which mainly 21427



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consists of four steps: (1) electrospinning of nano-CaCO3− PAN composite nanofibers; (2) preoxidative stabilization of CaCO3−PAN nanofibers in air; (3) carbonization of electrospun CaCO3−PAN and pyrolysis of embedded CaCO3 to CaO; and (4) acid washing to remove nano-CaCO3 or asformed nano-CaO. PAN is one of the best precursors for electrospinning due to its good spinnability and high carbon yield.25,26 Commercially available nano-CaCO3 is compatible with PAN-DMF solution and can be easily co-electrospun to form nano-CaCO3−PAN composite nanofibers. During the thermal treatment, when the temperature was higher than 670 °C, which is the pyrolysis temperature of CaCO3, CO2 was released and squeezed out from the inside of carbon matrix. The released CO2 would be an activation agent which consumed the partial carbon materials in the nanofibers according to the equation CO2 + C → 2CO and therefore formed mesopores.27,28 After the removal of the as-formed CaO, hierarchical porous carbon nanofibers with both macro- and mesopores were generated. These macropores are interconnected by the as-formed winding mesopores to form hierarchical porous structure, which is illustrated in Scheme 1. When the carbonization temperature is lower than the 670 °C, the embedded CaCO3 would not be able to convert to CaO; only macroporous carbon nanofiber was formed after the removal of the embedded nano-CaCO3. Figure 1a depicts the SEM image of the electrospun nanoCaCO3−PAN composite nanofibers. The nano-CaCO3−PAN

decomposition of organic and carbon densification during the carbonization process. The morphology of the precursor composites remained as continuous and uniform nanofibers with smaller diameter, as shown in Figure 1c. After the removal of nano-CaCO3, large macro- and mesoporous structure was formed, as seen in Figure 1d and its inset. In order to determine the influence of the nano-CaCO3 on the oxidative stabilization process, we performed TG-DTA on electrospun nano-CaCO3−PAN nanofibers in air. As shown in Figure S1a, bare PAN nanofiber experienced two different periods of transformation: oxidative stabilization at ca. 300 °C and consumption of carbon in air at temperature above 500 °C. When nano-CaCO3 was incorporated into PAN nanofibers, a new stage occurred at the temperature above 650 °C where the incorporated nano-CaCO3 transferred into nano-CaO and released CO2. As PAN nanofibers are converted to carbon nanofibers, oxidative stabilization is a critical requirement for obtaining dimensional stability during the high-temperature thermal treatment.13,25,29,30 As shown in Figure S1b, all samples exhibited a strong exothermic peak corresponding to complex and multiple chemical reactions (e.g., dehydration, cyclization, dehydrogenation, cross-linking). Linearly up-shifted exothermic peaks with increasing nano-CaCO3 content indicated that incorporated nano-CaCO3 might inhibit the oxidative stabilization reactions to some extent. The carbon nanofibers will not be obtained when the weight ratio of nano-CaCO3 to PAN exceeded 2, probably because too much nano-CaCO3 hindered the oxidative stabilization reactions and PAN decomposed without forming the thermosetting structure. Hence, in order to fabricate macro- and mesoporous carbon nanofibers, the weight ratio of nano-CaCO3 to PAN for electrospinning should be controlled below 2. Figure 2 shows the TEM images of as-prepared carbon nanofibers with as-formed CaO embedded into the carbon nanofiber matrix (Figure 2a, inset) and subsequently formed hierarchical pores inside the carbon nanofibers (Figure 2a). The as-formed CaO distributed uniformly throughout the carbon nanofiber, and these nanoparticles extended onto the surface of the carbon nanofibers causing rough surface of the carbon nanofibers. After removing the as-formed CaO via HCl leaching, the whole nanofiber became porous. Figure 2b shows the magnified TEM demonstrating the pore structure inside the carbon nanofiber matrix after CaO removal. The interconnected macropores are randomly located inside the carbon nanofiber, and the size of the macropores corresponds exactly to the nano-CaCO3 particle size shown in the Figure 2b inset. The porosity of the hierarchical macro- and mesoporous carbon nanofibers were determined by the N2 adsorption/ desorption method. Figure 3a shows the N2 adsorption/ desorption isotherms of G-CNFs-650 and G-CNFs-900 samples. The N2 adsorption/desorption isotherms of both samples show a sharp capillary condensation step at high relative pressures (P/P0 = 0.85−0.99), indicating a relatively large pore size. The hysteresis loop of sample G-CNFs-900 at a relative pressure range of 0.45−0.85 suggests the presence of mesopores, which can be classified into type H4 according to the IUPAC nomenclature. This is because when the temperature reached over 670 °C, the released CO2 squeezed out from the inside of the carbon nanofibers and reacted with carbon under high temperature according to the aforementioned equation, leaving winding mesopores insides the carbon nanofiber matrix. Such a CO2-induced pore generating

Figure 1. (a) SEM image and (b) its magnified SEM image of the electrospun composite nanofibers of nano-CaCO3 and PAN (nanoCaCO3−PAN). (c) SEM image of composite carbon nanofibers treated at 900 °C in Ar before the removal of nano-CaO (nano-CaO− CNFs-900); the inset in (c) shows the magnified cross section without macropores. (d) SEM image of porous carbon nanofibers after removal of nano-CaO (G-CNFs-900); the inset shows the magnified cross section with macropores.

nanofibers exhibited cylindrical morphologies and were continuous and long with an average diameter of about 1.3 μm. The CaCO3 nanoparticles of about 50 nm were uniformly dispersed inside the PAN matrix, causing convex surface of composite fibers (Figure 1b). After thermal treatment at 900 °C in argon, the diameter of the carbon nanofibers is reduced to 900 nm compared with nano-CaCO3−PAN composite nanofibers in Figure 1b. The size reduction was due to the 21428



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Figure 2. (a) TEM image of as-formed G-CNFs-900; inset shows the as-obtained CaO−CNFs-900. (b) Magnified TEM image of G-CNFs-900 shows the interconnected macroporous structure; inset shows TEM image of nano-CaCO3.

Figure 3. (a) Nitrogen adsorption/desorption isotherms of G-CNFs. (b) Pore-size distributions of G-CNFs samples calculated by the BJH method.

Figure 4. (a) TEM images of Pd/G-CNFs-900 and (b) its magnified TEM image; inset shows the size distribution of Pd nanoparticles of Pd/GCNFs-900.

proves that the developed mesopores are mixed up with macropores, resulting in this distorted hysteretic curve. The GCNFs-900 sample also has a total macropore volume of 18.7 cm3 g−1 and a porosity degree of 50.4% determined by the mercury porosimetry method. Such high pore volume and porosity are very useful in adsorption application of oil spill cleanup for instance, which are being investigated in an ongoing study. The as-synthesized hierarchical porous carbon nanofibers combine the structures of one-dimensional, meso- and macroporous structure and thus have potential applications in liquid catalysis. G-CNFs-900 was selected as a support for palladium catalyst. As is shown in Figure 4, numerous Pd nanoparticles were uniformly anchored inside G-CNFs-900 matrix. The Pd particle size of 3.5−4 nm determined from the histogram of particle size distribution is relatively small; this is probably due to the hierarchical porous carbon nanofiber

mechanism can be further confirmed by the presence of “explosive expansion joint” (Figure S2). Such an explosion inside carbon nanofiber was caused by agglomerates of nanoCaCO3 particles, which produced too much CO2 within a small region of the carbon nanofiber matrix. The corresponding pore size distribution data calculated from desorption branch of nitrogen isotherms by the BJH method (Figure 3b) shows the pore size mainly distributed at 4.2 nm. In the case of G-CNFs650 sample, the carbonization temperature was not high enough for the pyrolysis of CaCO3 to CaO, thus leaving only macropores inside the carbon nanofiber matrix after removal of the embedded nano-CaCO3. The BET surface area of G-CNFs900 is 123.0 m2 g−1, which is far greater than the value (29.1 m2 g−1) of sample G-CNFs-650. It should be noted that the capillary condensation of G-CNFs-900 begins from the relative pressure of 0.45 all the way up to 0.99, which is slightly distorted and deviated to higher relative pressure. This in fact 21429



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Figure 5. (a) XRD pattern, (b) XPS survey spectra, XPS of (c) N 1s region, and (d) Pd 3d pattern of Pd/G-CNFs-900.

Table 1. Suzuki Reactions Catalyzed by Pd/G-CNFs-900a

entry

R1

X

R2

solvent

T (h)

yieldb (%)

TOF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

H H H CH3O CH3CO H H H CH3O CH3O CH3O CH3CO CH3CO CH3CO CH3CO CH3CO H H H CH3O CH3CO CH3CO

I I I I I Br Br Br Br Br Br Br Br Br Br Br Cl Cl Cl Cl Cl Cl

H CH2CH CH2CH H H H CH2CH CH2CH H H H H H H H H H H CH2CH H H H

EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH DMF EtOH EtOH EtOH DMF DMF EtOH DMF EtOH EtOH EtOH DMF

0.5 1 3 1 0.5 2 2 3 1 3 1 1 2 3 1 2 1 3 3 3 3 3

99.5 55.4 84.3 98.2 99.0 96.1 12.4 30.9 48.7 78.9 97.0 46.8 60.0 81.3 84.3 96.2 10.8(22) 53.0 12.0(20)

278.7 77.6 39.3 137.5 277.3 67.3 8.7 14.4 68.2 36.8 135.8 65.5 42 37.9 118 67.4 15.2(30.8) 24.7 5.6(9.3)

43.4

20.3

a

Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid (1.5 mmol), K2CO3 (1.5 mmol), catalyst (5 mg, Pd: 7.6 wt %), solvent (10 mL), reflux for 0.5 h. bThe number of yields in parentheses was obtained by adding 0.1 mmol KI to the reaction systems. Elsewhere is the same meaning.

structure which provided high surface areas (Figure 3a) and the residual nitrogen species (Figure 5c) that is helpful for anchoring noble metal nanoparticles. The crystallinity of the Pd/G-CNFs-900 was investigated by XRD, and the XPS analysis was also recorded in Figure 5 to determine the oxidation state of surface elemental in materials. As shown in Figure 5a, the diffraction peak of Pd/G-CNFs900 at around 25° results from the (002) plane of graphite layers. Characteristic diffraction peaks at 40.1°, 46.8°, and 68.1° correspond to the (111), (200), and (220) crystal planes of

metallic Pd for Pd/G-CNFs-900. According to the Debye− Scherrer formula, the average size of Pd nanoparticles is estimated to be 4.0 nm, which is well consistent with the TEM results shown in the Figure 5b inset. The wide survey spectra of Pd/G-CNFs-900 showed Pd XPS and Auger transitions and peaks corresponding to C, O, and N (Figure 5b). No trace of Ca was detected, suggesting that the acid washing removed the as-formed CaO completely. PAN is a nitrogen-rich precursor, and the nitrogen species are usually hard to remove during thermal treatment. The 21430



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commercially available nano-CaCO3 is relatively low-cost, nonpoisonous, and can be easily removed by HCl. This is a low-cost, facile, and scalable method to prepare 1D hierarchical porous carbon nanofibers. The porous carbon nanofibers combine the advantages of 1D structure, meso-, and macropores; when being loaded by Pd nanoparticles, the catalyst showed high activity in various Suzuki cross-coupling reactions.

presence of nitrogen species facilitates the loading of Pd nanoparticles onto the support and enhances the capacity for adsorbing metal ions in aqueous solution due to the binding between nitrogen species and metal.31,32 The high-resolution N 1s peak in XPS is displayed in Figure 5c. The XPS spectrum of N 1s region consists of three different peaks. The signals at 398.0, 399.8, and 400.5 eV are signed to pyridinic nitrogen, pyrrole type nitrogen, and pyridine type nitrogen, respectively.33 Figure 5d shows the deconvolution of the Pd 3d5/2 spectra. The Pd 3d5/2 spectra were separated into two components, which can be attributed into metallic Pd (335.8 eV) and palladium oxide (337.9 eV). The above result indicates that the oxidation state of Pd species on G-CNFs-900 was a mix valence state of Pd(+2) and Pd(0). Suzuki cross-coupling reactions with various substrates were carried out as model reactions to evaluate the performance of as-formed Pd/G-CNFs-900.34 The loading of palladium on the G-CNFs-900 was 7.6 wt % as determined by ICP analysis. The reactions were conducted using either ethanol or DMF as the solvent and K2CO3 as the base. As shown in Table 1, the catalyst showed excellent activity in various reactions. Besides usually reactive iodobenzene (Table 1, entry 1), bromobenzenes also showed good results, especially when DMF was used as solvent. As for chlorobenzene, a small amount of KI added to the reaction system could improve the yields to some extent (Table 1, entries 17 and 19). The effect of substituted groups in substrates was investigated. The conversions of substituted substrates were lower than unsubstituted ones. Changing solvent to DMF could also accelerate the reaction to obtain a satisfactory yield due to the high reflux temperature of DMF. As a comparison, we compared the results in this work with those reported elsewhere over Pd-based catalysts. The results are listed in Table S1 with iodobenzene reacting with phenylboronic as a model example. Pd/G-CNF-900 exhibited higher catalytic activity to other Pd-based catalysts in Suzuki coupling reactions, and the observed higher activity might be due to the relative smaller palladium nanoparticles and the hierarchical porous structure. Reuse of the catalyst is a crucial requirement for any practical application in terms of cost and environmental protection. As illustrated in Figure S3, the catalyst can be reused for five times with no decrease of conversion and selectivity. Another key factor to be investigated is the stability of the catalyst, i.e., the leaching of active species into the reaction mixture. The catalyst analyzed by ICP after five recycles showed only 0.2% loss of palladium species. The high catalytic activity and stability of the as-prepared Pd/G-CNFs-900 may be ascribed to the hierarchical porous structure and the residual nitrogen species. The interconnected macropores combined with the 1D carbon structure facilitate the mass transport, and the attached mesopores provide high surface area for contact of reactants with palladium nanoparticles. Besides, the residual nitrogen in the carbon support may also contribute to more anchoring sites that result in small palladium nanoparticles and high palladiumsupport affinity.

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ASSOCIATED CONTENT

S Supporting Information *

TG and DTA thermogram of nano-CaCO3 incorporated PAN nanofibers and TEM image of the “explosive expansion joint” for G-CNFs-900 and catalytic performances of different Pdbased catalysts and recyclability test of Pd/G-CNFs-900 in Suzuki cross-coupling reaction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +86-10-62557908 (W.-G.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from the National Basic Research Program of China (2009CB930400), National Natural Science Foundation of China (NSFC 21121063), and the Chinese Academy of Sciences (KJCX2-YW-N41).

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REFERENCES

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4. CONCLUSIONS In summary, we produced hierarchical macro- and mesoporous carbon nanofibers using commercially available nano-CaCO3 as a dual purpose template. Specifically, CO2 produced by the nano-CaCO3 templates’ pyrolysis can generate mesopores insides the 1D carbon nanofibers, and the macropores can be generated after the removal of the as-formed CaO by HCl. The 21431



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